THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

DON LEMON, CNN ANCHOR: For nearly 27,000 runners and the hundreds of thousands supporting them, this was the big race, the Boston marathon. But what started with a flush of energy and anticipation ended in horror just short of the finish line changing lives in an instant.

This is a story of those who barely escaped harm, of those who weren't that lucky, of those who did what they could to help.

Drew Griffin begins our Special Report now.

(BEGIN VIDEOTAPE)

UNIDENTIFIED MALE: I have multiple people down here, OK? I don't know what the cause is. Stand by.

UNIDENTIFIED FEMALE: Something just blew up!

UNIDENTIFIED MALE: We've had an attack. Oh, my God.

UNIDENTIFIED MALE: Get back. Get back.

UNIDENTIFIED MALE: We need as many people as you can from the medical tent.

DREW GRIFFIN, CNN INVESTIGATIVE CORRESPONDENT (voice-over): The Boston marathon turns from triumph to tragedy. The FBI zeros in on the terrorists behind the bombing.

RICHARD DESLAURIERS, FBI SPECIAL AGENT IN-CHARGE: After a very detailed analysis, a photo video and other evidence, we are releasing photos of these two suspects.

GRIFFIN: Hours later, a suspect spotted. Then, a shootout that leaves one suspect dead and another on the run.

There's a huge, huge police presence here.

A city under lockdown and on the edge.

GOV. DEVAL PATRICK (D), MASSACHUSETTS: We're asking people to shelter in place, to stay indoors with their doors locked.

GRIFFIN: After a massive manhunt --

UNIDENTIFIED FEMALE: We have had hundreds of law enforcement officers who have zeroed in on the man they believe is the suspect. They have him cornered.

GRIFFIN: The suspected bomber captured alive. It was a week of tragedy that began with celebration.

It was going to be a great day in Boston. The winter pounding was over. Even better, it was Patriots day, an annual holiday for Bostonians.

JONATHAN ELIAS, REPORTER, WBZTV: I think for everybody, I think Patriots day stands for something.

GRIFFIN: Jonathan Elias was reporting for WBZTV on April 15th.

ELIAS: its kind like the fourth of July for Massachusetts. People celebrate, you know, who we are, where we are, and what (INAUDIBLE).

STEVE SILVA, REPORTER, BOSTON GLOBE: There is nothing else like it.

ELIAS: Boston globe reporter Steve Silva was in the mix before dawn at the starting line of the day's biggest event, the Boston marathon.

UNIDENTIFIED MALE: Part of the American sport scene since 1897.

SILVA: It was a chilly morning, 30 degrees, people starting to warm up, unlike last year where it was 80 degrees. So, thinking, the weather's not going to be a story this year. What's going to be the story?

GRIFFIN: For pediatric resident, it would be the story of giving back.

DOCTOR NATALIE STAVAS, PEDIATRIC RESIDENT: It was a very special day for me. I had decided to raise money for Boston medical center pediatrics.

GRIFFIN: Doctors told Chris that this was a day her daughter might not ever live to see.

CHRIS: They said her prognosis was, you know, maybe she'll be 10 if we were lucky.

GRIFFIN: The mido chondrial disease that weakened Kayla's cells made her terribly ill.

CHRIS: Those first few years were hard, she needed to have medical procedures, time in the hospital, lots of pneumonias.

GRIFFIN: For the Kay girls, it had been a tough go. Kayla's health challenges, two benign brain tumors for Chris, the lost of Kayla's father, Chris' husband to a heart attack at age 42. But this would be a joyful day. A day to make history as a mother, daughter, wheelchair team.

CHRIS: This was the first time in the 117 years with the Boston marathon that a female duo team actually entered and was going to complete the race. Well, if we're going to start this, we're going to finish it because one shot to do it and we're going to do it.

GRIFFIN: Natalie, too, had a special running companion.

STAVAS: I was with my father who was running the marathon with me. He had put in a lot of hours and dedicated a lot of time to training.

GRIFFIN: By the time they reached the starting line, the air was electric.

STAVAS: The pulse of the crowd at the starting line is just -- there's so much excitement and happiness.

CHRIS: When we got to Hopkinton and she got in her chair, you could just see the excitement. And she was just hugging people and high- fiving people and any, you know, any opportunity to kiss somebody, she was.

GRIFFIN: Wave after wave of runners started the race. Nearly 27,000 people running for the finish line.

SILVA: We did our, you know, our duty at the starting line, got everybody off and running and I jumped in the car with my intern and we headed for the finish line.

GRIFFIN: On the course, Natalie was midway through her 11th marathon but this one was different.

STAVAS: I remember the crowd and the energy and the enthusiasm and the excitement and the love and there's -- there's just something not -- that I've never experienced before that I experienced here in Boston.

CHRIS: Along the course we saw friends and family and, you know, genuine strangers coming up to us and just saying, you know, this is so inspirational. This is wonderful.

UNIDENTIFIED MALE: La Lisa (ph) is the 2013 Boston champion.

GRIFFIN: By 2:10 p.m., the elite runners were longed finished and crowds at the marathon were growing.

Among them, these two men looking like normal spectators, their dark plans undetected. They made their way to the finish line, setting their backpacks down in the crowd. Steve Silva and John Elias were standing nearby watching the finishers.

SILVA: You see cheers, you see people yelling "yes"! There's a lot of raw emotion at the finish line. A lot of those are people we know, people in our the neighborhood.

ELIAS: We're literally rolling of people coming across the line and high-fiving them and clapping and cheering them on. Everybody cheers them on.

GRIFFIN: Natalie stay this knew she was close. STAVAS: So there's this marker that every runner looks for in a race and if you're a marathon, you know what it means. It says, you have one mile to go. And my dad and I had passed that marker.

GRIFFIN: Chris and boyfriend Brian were already within sight of the finish line.

CHRIS: The crowds were just absolutely crazy and, you know, everybody was cheering and Kayla's doing her hill shaky shake.

GRIFFIN: Steve was rolling on the finishers. Then --

SILVA: Same time, just the blast, flash of fire, and the smoke. I could see it through the camera.

ELIAS: And when it went off, it literally was deafening and you feel your hair going up and your body tremble.

CHRIS: Brian had been hit with something because I could see the blood on his shoulder and he just instinctively in mess, split second grabbed the stroller and said, we've got to go.

GRIFFIN: Around the corner, Natalie did not know what had just happened.

STAVAS: We thought, fireworks, this is for us, you know, celebrating the finish of the marathon.

UNIDENTIFIED FEMALE: Something just blew up!

GRIFFIN: Twelve seconds later, another blast.

STAVAS: Pretty quickly we realized it was something that was not expected and something had gone horribly wrong.

ELIAS: Grandstands were empty. People were sprinting and running around and everything felt like it was in slow motion.

STAVAS: You're coming down the final stretch and it's a herd of runners and there's so much momentum in that herd and everyone stopped.

SILVA: There was a little bit of a dead silence right after it happened.

ELIAS: There weren't screams for help. There weren't people crying. Everybody had their mouths kind of open like they wanted to say something or wanted to scream, and nothing was coming out. And their eyes were wide open and they were in far away transits as though they were in complete shock.

GRIFFIN: Chris and Kayla, Natalie and her dad were all safe. Hundreds of others were not as lucky.

UNIDENTIFIED MALE: It was a process not fast enough for me to say, wait a minute, you know, that guy's leg is gone, or you know, that woman doesn't have a leg or where is her arm?

UNIDENTIFIED MALE: It was scattered, it felt like a 30 feet cross of just carnage, blood, body parts.

GRIFFIN: Then, what started as a race to a finish became a desperate race against time to save the lives of the gravely wounded.

Next, Dr. Sanjay Gupta on the fight for survival.

STAVAS: I was leaning over him and he grabbed me and he said, doctor, am I going to be OK?

(END VIDEOTAPE)

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(BEGIN VIDEOTAPE)

DOCTOR SANJAY GUPTA, CNN CHIEF MEDICAL CORRESPONDENT (voice-over): Four hours and ten minutes into the Boston marathon, the peak finish time of the race. Thousands of runners were about to reach mile 26. Then, the unimaginable. A bomb packed with nails and bbs rips into the crowd cheering near the finish line.

UNIDENTIFIED MALE: I have multiple people down here, OK? I don't know what the cause is. Stand by.

UNIDENTIFIED MALE: We need help from the medical tent. Get as many people up here as you can from the medical tent.

UNIDENTIFIED MALE: It was like, oh, my God, it actually happened. Most people didn't know what it was. They just heard the explosion. I knew it was an IED. There is no question in my mind.

GUPTA: (INAUDIBLE), an Iraq war veteran and registered nurse was volunteering in the medical tent.

UNIDENTIFIED MALE: I was convinced there was going to be another because what they used to do overseas sometimes is one would go off and then they would wait a little while. People would go to the aid of whoever was injured and set they would set off a second one to maximize their damage.

GUPTA: Just 12 seconds later, another explosion.

UNIDENTIFIED MALE: We have had an attack.

STAVAS: It was a deep feeling that almost just penetrated you to the core.

GUPTA: Dr. Natalie Stavas was running the race with her father.

STAVAS: I could immediately sense that something was terribly wrong and chaos broke out almost immediately. They tried to stop us and they were actually trying to barricade the rest of us back. I went over the barricade, actually I would swim. Police officers were yelling at me, stop, you must stop. And I kept going. Finally, as a police officer actually grabbed my arm and he said, ma'am, ma'am, you must stop. And I said, I'm a pediatric physician. I'm a pediatric physician. You have to let me go. You have to.

GUPTA: Dr. Stavas sprinted towards the finish line and straight into a war zone.

STAVAS: The crowd screaming and people screaming in pain and blood has a very distinct smell and you know that you're facing grave, grave tragedy if you -- if you're feeling that and you're sensing that. That you can smell the blood.

I went to the next woman who was ahead of me and right in the middle of Boylston street and she was laying on her back and I said, let me help you. Let me help you. I'm a doctor. And she showed me the wound and it was a large wound to the groin. Oh, it was a horrible wound. And I yelled, we need to get this woman in the ambulance now.

GUPTA: Dr. Stavas raced from one victim to the next.

STAVAS: Another young man, grabbed me and he said, doctor, am I going to be OK? And he was the first person that I looked at in the in the eyes and said, you're going to be OK.

STEVEN SAGATORI, REGISTERED NURSE: I saw volunteers running towards the finish line, wheelchairs coming over the barricades to get to the wounded.

GUPTA: Registered nurse Steven Sagatori had been handling ruin teen blisters and dehydration in the medical tent. Suddenly, dozens of victims now need his help.

SAGATORI: My first patient actually was a young man in a wheelchair. We brought him in, got him in a stretcher, and he went right out to the hospital. I went back out to start getting -- to bring the next person in and that's when I saw the stretcher with the physician on one side and an EMS coming in and a physician was doing CPR.

GUPTA: It was 29-year-old Krystle Campbell.

SAGATORI: He was just a beautiful young girl. She reminded me of my daughter, actually. And her wounds were horrific. You know, we worked very hard on her for a while until it was clear that it was -- it wasn't going to help the situation at all.

And at that point we were feeling for a pulse and then the cardiac rhythm on the monitor was such that she was not going to survive and at that point the physician, you know, pronounced her said she was dead.

GUPTA: Dead in his arms.

SAGATORI: We just kind of all -- we just let out this guttural scream, you know. But that was it for that moment because we had to go on to the next patient. GUPTA: For the next 45 minutes, Steven worked triage on the front lines. Dozens were rushed to the emergency room under code red, meaning their chances of survival uncertain.

GUPTA: Trauma surgeon Dr. David King crossed the finish line moments before the second explosion. As the bodies were rushed from the medical tent to the emergency room, Dr. King raced to mass general and spent the next 40 hours performing life-saving operations.

I mean, it's extraordinary. Within 90 seconds you said you were essentially on the way to the operating room with these patients?

KING: Yes. The injuries were so horrific and so obvious, when bleeding is massive, it doesn't take a lot of figuring to understand that something needs to be done quickly.

GUPTA: You had sort of a significant milestone today with regard to your patients, critically injured patients. But what happened today that was significant?

KING: Our last patient who was on a breathing machine today had their breathing tube removed. That doesn't mean we've won the game but it's a really big step in the right direction.

GUPTA: And it's a miracle for Boston as the heroism of the first responders saved 180 lives. Still, Steven Sagatori can't stop thinking about the ones they lost.

SAGATORI: I'm a nurse by profession so death is part of your profession but you expect it when you're at work. In this case, you know, everyone's out having a good time with a marathon and people are not dead. But I haven't been able to put Krystle away. I haven't been able to put down in the show, I haven't been that under a box and throw that away. I can't get it out of my mind right now. I know with time it will but right now I think about it all the time.

GUPTA: Coming up --

GRIFFIN: At least a couple dozen shots --

GUPTA: Drew Griffin investigates who was behind the horrific attack and the dramatic manhunt for the suspects.

(COMMERCIAL BREAK)

UNIDENTIFIED MALE: Any people up there in the medical tent.

GRIFFIN (voice-over): Even as first responders struggled to save lives, investigators began digging for evidence. As work of the massive crime scene got under way, Boston's police commissioner stood resolute.

EDWARD DAVIS, COMMISSIONER, BOSTON POLICE DEPARTMENT: We will turn every rock over to find who is responsible for this.

GRIFFIN: The criminal investigation pushed ahead painstaking work, the components of two bombs scattered across the streets for blocks. Each particle would need to be found, cataloged, collected. Twenty- seven hours after the blasts, these crime scene photos began circulating showing pieces of the bomb, a partial timing device, and a possible clue.

RICHARD DESLAURIERS, FBI SPECIAL AGENT IN CHARGE: Among items partially recovered are pieces of black nylon which could be from a backpack and what appeared to be fragments of bbs and nails possibly contained in a pressure cooker device.

GRIFFIN: At least one of the bombs was packed inside a Spanish-made widely available household pressure cooker. A six-liter stainless steel pot. Its mangled pieces found in the wreckage, a piece of its lid found on a rooftop.

JULIETTE KAYYEM, FORMER HOMELAND SECURITY OFFICIAL: This device is on the web. It is something that anyone could make if they wanted to and if they took the time and resources.

GRIFFIN: Juliette former homeland security official, predicted the would-be captured because of being caught on camera.

KAYYEM: There's no way that someone planning this would have thought that they wouldn't have been seen by some camera.

GRIFFIN: And she was right.

DESLAURIERS: We are releasing photos of these two suspects.

GRIFFIN: Late Thursday, a big break in the case. FBI officials released these pictures and video of two suspects, suspect one and suspect two. FBI officials say the suspect with the white cap was seen on video dropping off a backpack shortly before one of the two bombs exploded.

Just hours later, around 10:00 p.m. Thursday, one of the suspects is captured on a surveillance camera. Police say it is one of the suspected marathon bombers in the white hat. Later identified as 19- year-old Dzhokhar Tsarnaev.

Minutes later, shots are fired at MIT. A campus police officer, Sean Collier, is found in his cruiser with multiple gunshots. He has pronounced dead at the hospital. Not long after, two men carjack a Mercedes SUV. Watertown police spot that car and then a high-speed chase.

COLONEL TIMOTHY ALBEN, MASSACHUSETTS STATE POLICE SUPERINTENDENT: During the course of that pursuit, several explosive devices were discharged from the car at the police officers. In the exchange of the gunfire, we believe that one of the suspects was struck and ultimately taken into custody.

GRIFFIN: A transit officer was shot and wounded in the gunfight but police did shoot one man, 26-year-old Tamerlan Tsarnaev, identified as suspect number one and Dzhokhar Tsarnaev's older brother. He later dies at a hospital, strapped to his body, bombs. Dzhokhar Tsarnaev is still on the run and an American city is put on lockdown.

PATRICK: There is a massive manhunt under way.

GRIFFIN: Full neighborhoods are evacuated and Watertown looks like a ghost town while a massive manhunt begins for the second brother, the nation begins to learn just who these men are.

The older brother who was married and had a young daughter studied engineering at Bunker Hill community college. He took the year off to train as a Boxer. The younger brother is a student at The university of Massachusetts, Dartmouth. Friends say he was a wrestler and a lifeguard. It turns out the family comes from Chechnya, the breakaway soviet republic which has for decades been racked by violence from extremists who have fought against Russian control. The family came to the United States more than a decade ago. Zubeidat Tsarnaeva is the mother of the two suspects.

ZUBEIDAT TSARNAEVA, MOTHER OF THE SUSPECTS: Nobody talks about the terrorism and he never, he never told me that he would be like on the side of jihad.

GRIFFIN: Anzor Tsarnaev is the suspects' father.

ANZOR TSARNAEV, SUSPECTS' FATHER (through translator): Someone framed them. I don't know exactly who did it but someone did and being cowards they shot the boy dead. There are cops like this. It's all because I'm afraid for my son and his life. They should arrest him, maybe, and bring him but alive, alive, and justice should decide who is right and who is guilty.

GRIFFIN: Friends from the Boston area who knew them say they are surprised.

ERIC MERCADO, HIGH SCHOOL FRIEND: Dzhokhar I remember was a cordial kid, a happy go lucky kid. There was nothing to indicate that he would, you know, to be part of a terrorist plan.

YANNA BLUE (PH), FORMER CLASSMATE OF DZHOKHAR TSRNAEV: He was a regular teenager to me.

GRIFFIN: Yanna Blue (ph) went to both carter and high school with Dzhokhar Tsarnaev.

BLUE (ph): He was quiet, you know, just a regular teenager, didn't suspect anything. He was on the wrestling team. He went to parties with, you know, other students. Yes. He went to the prom. Regular, pretty much. Nothing that caught me off guard that made me suspect anything, which is why I'm really surprised about what happened because I didn't expect that at all from him.

GRIFFIN: More recently, he used twitter often, even after the bombing. Among his tweets on April 15th, "ain't no love in the heart of the city, stay safe people." And a university official says Tsarnaev was on the campus of the university of Massachusetts' dark every day after the attack until Thursday where he attended classes and parties in the dorms.

Other details began to emerge. An FBI official told CNN that Tamerlan Tsarnaev was interviewed by agents in 2011 at the request of a foreign government. That government suspected that he might have ties to extremist groups. But the FBI says nothing was found.

As the hunt continued into Friday night, even the most veteran law enforcement officials could not believe how big it was.

THOMAS SHAMSHAK, FORMER BOSTON POLICE CHIEF: This city has never seen anything like this going back to the era of the Boston strangler in the early '60s. Right now there is total paralysis in the city of Boston.

GRIFFIN: What was that? Was that guns?

But it would soon end in Watertown.

I just heard what sounded like multiple assault rifle shots to me.

It sounded like police emptying their weapons, rapidly fired and then all of a sudden it stopped.

SUSAN CANDIOTTI, CNN NATIONAL CORRESPONDENT: There was a lot of commotion over in this area. We just saw more polices vehicles with their lights going.

UNIDENTIFIED MALE REPORTER: They have the suspect. They believe it is the suspect. They know exactly where he is. They have cordoned off a section of Watertown.

UNIDENTIFIED MALE: Please, clear out, OK?

GRIFFIN: By 8:15 p.m., law enforcement officials say a person believed to be Dzhokhar Tsarnaev is cornered, hiding on a boat in a backyard. Finally, at 8:46 p.m., he's in custody and taken to the hospital with gunshot wounds. Minutes before 9:00 p.m., this tweet from the Boston police department, captured.

CROWD: USA! USA! USA!

GRIFFIN: Coming up, Don Lemon with stories of heroism and compassion.

(COMMERCIAL BREAK)

DON LEMON, CNN ANCHOR, NEWSROOM (voice-over): Boston, a city of rivers and bays. A city of ethnic neighborhoods. A city rich in American history. And on April 15th, in the midst of mayhem, a city of self-less compassion.

UNIDENTIFIED MALE: We actually had a pool of blood in the middle of our store. There was a woman actually grabbing spools of yarn in our store wrapping people to cut off the circulation so they wouldn't bleed out. UNIDENTIFIED MALE: Oh, my God.

LEMON: People whose impulse was to run toward rather than away from death and danger, some of their deeds caught on video or in photos.

CARLOS ARREDONDO, ACTIVIST: I just run to see how I can help -- see what can I do.

Carlos Arredondo was at the finish line cheering on runners when the bomb went off. He had come to the Boston marathon to honor his sons.

This is my beautiful boys and I call this picture my American dream. They are my angels.

Carlos came from Costa Rica to the United States more than 30 years ago. His sons were born in the U.S. and he says both wanted to serve their country.

ARREDONDO: This is Alexander Arredondo who served two tours in the Iraq. At the age of 28-year-old, a sniper shot took his life.

LEMON: Very handsome guy. He does the uniform proud. And who is this?

ARREDONDO: This is Brian right here. He die of suicidal in December 19th, 2011. He was often a lot -- he lost his brother.

LEMON: Carlos has been fighting his own demons. Attempting suicide after receiving news that his son Alexander was killed in Iraq. He came to the marathon to hand out American flags and to cheer on two groups, one honoring the fallen soldiers, the other to raise awareness of and prevent suicide.

ARREDONDO: We had a good reasons to be there, to share, to honor, and to remember.

LEMON: When the bomb went off, the first thing Carlos did was snap photos.

ARREDONDO: I was holding my camera in my hand, you know, even though I was there and shocked, I managed to take four pictures.

LEMON: Carlos says he quickly put his camera away and went across the street to help victims but there were barriers in the way.

ARREDONDO: So we needed to break it down in order to come and help and that was the first thing we did.

LEMON: Carlos dropped to the ground to help Jeff Bauman who had lost both legs.

ARREDONDO: I look at his legs, I ask for help, he has somebody help and then another person come over. I grab a sweater from the ground and I rip it apart in between this person and myself we start stopping the bleeding from Jeff's legs. And then I pick up Jeff from the floor. I pick him up and I sit him in the chair and I told him to hold on, hold on, and I told the lady, we have to rush, we have to rush. Let's move. Let's move it. And then we continue moving along the road, under the finish line and then into the emergency tent which they want me to stop here, stop here, stop. Right here, right here. I say, no, I need an ambulance. I ended up taking him all the way to the ambulance, which I pick him up again and I put him in the bed and that's the last time I saw Jeff.

LEMON: And then you end up with this?

ARREDONDO: Well, this is the American flag that I was carrying when I went in to the attack and as you can see, it's all stained in blood and --

LEMON: Whose blood is this?

ARREDONDO: This must be the blood of the young man who I helped out because I was holding this flag and I was helping out this young man (INAUDIBLE) because I was holding this flag and I was covered with blood.

LEMON: But you handed out so many flags that day.

ARREDONDO: Yes.

LEMON: And you have one left?

ARREDONDO: This was the last I have.

LEMON: Do you feel like a hero?

ARREDONDO: No, I not feeling like a hero. I did pretty much what others been doing the same thing.

LEMON: And the man Carlos helped, Jeff Bauman, also became a hero. While in intensive care, he gave the FBI a description of the man who dropped the backpack at his feet.

BARACK OBAMA, PRESIDENT OF THE UNITED STATES: In the aftermath of the explosions where stories of heroism and kindness and generosity and love.

LEMON: Bostonians took to social media, offering any help they could. El Pelon Tacqueria A little Mexican restaurant near the bomb attack transformed itself into a gathering place, tweeting, open wi-fi, place to charge cell, or just don't want to be alone, food and drinks, pay only if you can.

JIM HOVEN, OWNER, EL PELON TACQUERIA: We have drinks, phone chargers, a quiet place to sit and on that day there was a shortage of those things.

LEMON: Just blocks from where the bombs went off, the restaurant did become a place of refuge.

HOVEN: The way that they were anxious. Mostly spectators, people who got separated from people they were with, just needed a place to sit down or a place to meet.

LEMON: For those wanting to take shelter in a home, "the Boston Globe" newspaper created a Google spreadsheet that was quickly filled by residents offering to take in complete strangers, strangers like Alie Hatfield from Kansas City, a runner left stranded after the race. She spoke to us via Skype.

HATFIELD: He said, I live right there. If you need anything, come and knock on my door. You can come inside if you need something. If you need something, a really nice woman brought us blankets. We huddled up and all sat really closely together but it was one of those things, too, where you couldn't just really relax.

LEMON: After an hour and a half of sitting on the sidewalk, another good Samaritan offered shelter.

HATFIELD: She had a beautiful living room and it was just so nice to sit on a couch. It was just a time that we could all calm down and reflect and check the news and try to feel safe. It reminds you that people do have love in their hearts. People out there, they really are good people. It's all the bad that we saw on Monday. And while there was so much tragedy, there was also so much love.

LEMON: So much love and so much loss.

LILIAN CAMPBELL, VICTIM'S GRANDMOTHER: She just used to say, take one day at a time, Nana, see what happens.

LEMON: Coming up, Gary Tuchman on lives lost.

CAMPBELL: She loved life.

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GARY TUCHMAN, CNN NATIONAL CORRESPONDENT (voice-over): There were no targets. No specific targets. Because everyone was targeted.

UNIDENTIFIED MALE: We always thought of them as one unit, just almost like singular, the Richards. They were always together anywhere they go. And you know, that's no longer the case.

TUCHMAN: 8-year-old Martin was killed. He loved the Boston bruins, the Red Sox, and Pictionary.

UNIDENTIFIED FEMALE: Every time I baby sit him, as soon as I came to the door, he came running towards me. And he just had a bright smile that nobody could ever forget. TUCHMAN: Martin's mother, Denise Richard, was a librarian at a school. She suffered severe head injuries. Martin's 7-year-old sister, Jane, who loves to dance lost a leg.

UNIDENTIFIED MALE: She's an Irish step dancer, loves it. And you know, everyone I've talked to, the one thing they said, if anyone's going to bounce back from this, it's Jane. She's a pistol. She's a tough kid. Don't think her dancing days are over.

TUCHMAN: Krystle Campbell was just a few weeks shy of her 30th birthday. She was there for the sheer spectacle, her grandmother says.

CAMPBELL: She just liked being in a crowd, seeing people, talking to them.

TUCHMAN: Lilian Campbell told our Chris Cuomo about their special bond ever since Krystle was a little girl.

CAMPBELL: I used to love to dress her up and put her hair in long curls and lots of bows on her hair. She loved it. She would go out prancing, proud as anything to school. And then in school the teachers would say, oh, Krystle, you look so beautiful. Who did your hair? My Nana did my hair.

TUCHMAN: Both Krystle and her best friend were injured.

SAGATORI: She had to have been very close to the explosion for the wounds she had.

TUCHMAN: Steven Sagatori, a registered nurse, was in the medical tent.

SAGATORI: We got her into the tent and got her stationary. I told her who I was, I was a nurse, and where she was, in the medical tent, and that we were working on her. And you never say you're going to be OK because you don't know that. So I didn't say those words to her but I did say they were trying their best.

TUCHMAN: When Krystle's father learned she was injured, he rushed to the hospital. At first he was told that she would live.

CAMPBELL: He said, Ma, Krystle had an operation and they had to take the vein out of one leg and put it in the other leg.

TUCHMAN: But the optimism soon turned to bewilderment and panic.

CAMPBELL: And he went in, took the covers down off of the girl, Krystle, and he said, that's not my daughter.

TUCHMAN: In a heartbreaking mix-up, Krystle had been confused with another patient. The family would learn Krystle had died much earlier at the medical tent near the finish line.

CAMPBELL: My son called me and said that Krystle's gone, Ma. I said, where did she go? Ma, Krystle passed on. And I said, no. No way. I thought she was going to be all right? No, Ma, they got the names mixed up and it was her girlfriend that was in the bed, not Krystle.

TUCHMAN: The confusion made the grief unbearable.

UNIDENTIFIED FEMALE: We are heartbroken at the death of our daughter Krystle Marie.

Krystle's mother scarcely able to speak.

UNIDENTIFIED FEMALE: Everybody that knew her loved her. She loved her dogs. (INAUDIBLE). She was always smiling and friendly. I couldn't ask for a better daughter. I can't believe this has happened. She was such a hard worker at everything she did. This doesn't make any sense.

TUCHMAN: Lu Lingzi died far away from home. She was from China attending graduate school, studying statistics at Boston University.

UNIDENTIFIED MALE: She had an uphill climb like a lot of foreign students that come here.

TUCHMAN: Eric (INAUDIBLE) was her academic adviser.

UNIDENTIFIED MALE: She had a lovely smile to her that was kind of half serious, half perplexed and half ready to laugh.

TUCHMAN: Lingzi had just taken her final exam to earn her degree. (INAUDIBLE) still has it waiting for a grade.

UNIDENTIFIED MALE: It's completely senseless. It's such a waste. It's such a waste of all the time and energy and dreams that she had and we'll never know what she could have done.

TUCHMAN: Alongside the broken hearts, people from all walks of life are part of the healing.

OBAMA: In the face of those who would visit death upon innocence, we will choose to save and to comfort.

TUCHMAN: Natalie Stavas, the marathon runner, plans to donate the medal she won for crossing the finish line to one of the victim's families.

STAVAS: They were there at the finish line to cheer me. Me, as part of the group across the finish line. And should I say that they -- that they died because of me?

TUCHMAN: Steven Sagatori, the nurse in the medical tent, is equally committed to taking care of runners and spectators.

SAGATORI: I will absolutely be there working with the medical staff. We can't rule our lives based on what some extremist is going to do.

TUCHMAN: Grieving families carry the heaviest weight.

CAMPBELL: She used to say, Nana, you're so strong. Stay strong. Don't give in to it. Don't fall down on your knees. Get up. TUCHMAN: All of Boston has vowed to get up, to honor the spirit of the runners and the city that cheers them on.

OBAMA: And this time next year on the third Monday in April, the world will return to this great American city to run harder than ever and to cheer even louder for that 118th Boston marathon. Bet on it.

(COMMERCIAL BREAK)

LEMON: April 15th, 2013, is a day the city of Boston will never forget, a day that the marathon turned to mayhem, but it was also a day of strength, resilience, and triumph over tragedy.